Numerous pharmaceutical drugs suffer from the disadvantage of being poorly soluble in an aqueous medium, thus having an insufficient dissolution profile to be able to be absorbed in systemic circulation following oral administration, consequently are less bioavailable. In order to achieve sufficient therapeutic effect, the therapeutic dose required to be administered must thus be increased in order to obviate this disadvantage. This particularly applies to compounds classified into Class II according to US biopharmaceutical classification system.
Atovaquone is a well-known antiparasitic and antipneumocystic drug, which is commercially available in different dosage forms and various doses since 1992, but the bioavailability of tablet formulations barely crosses 12% (23% under fat fed conditions) and for oral non-sized suspension bioavailability is about 23% (46-48% under fat fed condition). Indeed, due to its poor hydrosolubility, Atovaquone is poorly absorbed in the digestive tract and consequently its bioavailability is incomplete, irregular and often varies from one person to another. Fat meals help to dissolve the drug in the lipids present in the food. In order to achieve therapeutic levels of atovaquone, patient requires having sufficient fatty food, and most often, it is impractical due to condition/severity of the underlying disease nature.
Atovaquone (Formula I), chemical name being trans-2-[4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-1,4-naphthoquinone, is a hydroxy-1,4-naphthoquinone, an analog of Ubiquinone, with antipneumocystic and anti-malarial activity. It has previously been disclosed, for example, in European Patent No. 1,23,238 and 0675711 that Atovaquone is active (in animals and in vitro) against Pneumocystis (carinii) jirovecii, Plasmodia, tachyzoite and cyst forms of Toxoplasma gondii, and Eimeria spp., a causative agent for Coccidiosis. Further uses of Atovaquone for Cryptosporidiosis and Babesiosis are disclosed in European patent application no. 0496729 and U.S. Pat. No. 5,559,156 respectively.

For treatment as well as prophylaxis of malaria, proguanil hydrochloride is co-administered with Atovaquone. Chemical structure of Proguanil is given in Formula II.

Over the past 27 years, nearly 25 million people have died from HIV/AIDS which causes debilitating illness and premature death in people during their prime years of life and has devastated families and communities. The AIDS pandemic and the use of intensive immunosuppressive therapies for a variety of conditions, including before and after organ and tissue transplantation, have established a large population of immunocompromised individuals prone to reactivation of opportunistic pathogens, including Toxoplasma gondii, Pneumocystis (carinii) jirovecii and Eimeria spp., to name a few of the apicomplexan and non-apicomplexan organisms, particularly in the geographical areas with a high exposure rate to this pathogens. Atovaquone is well known in the art for its use against these pathogenic organisms in normal as well as immunocompromised patients.
Malaria is another infectious disease that causes severe morbidity and mortality with an estimated 300-500 million cases worldwide and more than 1 million deaths annually in sub-Saharan Africa alone and affected patients are of any age group. The disease is caused by another protozoan parasite of the genus Plasmodium, transmitted by mosquitoes. The most serious forms of malaria are caused by Plasmodium falciparum and Plasmodium vivax, but other species (e.g., Plasmodium ovale, Plasmodium malariae, and Plasmodium Knowlesi) can also infect humans. Control of malaria has been hampered by the spread of drug resistance in both the Plasmodium parasites and the Anopheles insect vector, and by the lack of an efficacious vaccine (Moorthy, V. S. et al., 2004. Lancet 363:150-156). In order to combat drug resistance and to improve antimalarial chemotherapy, a combination of antimalarials is commonly used, either simultaneously or sequentially. One such combination for the treatment of malaria which has previously been disclosed in WO9412164, U.S. Pat. No. 6,413,993, WO2009001367, and WO2009042960 is Atovaquone and Proguanil as hydrochloride salt (trade name: Malarone).
However, use of Atovaquone either alone or with proguanil, as a first line drug against pathogenic parasitic infectious diseases including malaria is severely hampered by the affordability of treatment due to the hefty price of Atovaquone despite its extreme safety profile, primarily linked to its poor aqueous solubility and thereby bioavailability, consequently very high doses of Atovaquone are required for treatment (Curative dose for malaria is 3 gm divided over three days, and ˜1.5-3.0 gm/day for other parasitic infections). There are numerous approaches reported in past for increasing the bioavailability of Atovaquone, however, the success is far from reach to make it as a first-line drug for treatment of pathogenic parasitic infections such as Malaria, especially in the third world.
A commercially successful strategy has been reported in U.S. Pat. Nos. 6,018,080 & 6,649,659 for improving bioavailability of Atovaquone. This patent describes the effect of micronizing Atovaquone together with cellulose carriers to improve Atovaquone solubility and thereby increase its bioavailability. This patent uses microfluidization process for preparing microparticle of 0.1-3 micron size of Atovaquone which can improve Atovaquone bioavailability almost double than solely micronizing the Atovaquone. The process comprises microfluidizing a suspension of atovaquone in a carrier such as HPMC, methylcellulose, etc., similar to preparation of solid dispersions in inert carrier polymers. Yet such microparticles of atovaquone are supplied as suspensions to provide the required daily dose to patients (1750 mg/day), and the bioavailability reaches 23% (with non-fat diet) and 46-48% with fat diet. However, the preparation method in that patent is not completely satisfactory in as much as it does not lead to complete bioavailability of the active ingredient, and suffers from several disadvantages. It should be especially noted that a fat diet alters the bioavailability of the micronized formulation more than double, however, should add wide variability due not only to fat content of the food, but also the capacity of absorption of fat between subjects may vary drastically, and thus therapeutic levels of Atovaquone may not been achieved. A possible reason for resistance to malaria treatment by Atovaquone is also discussed later which is due to this variability in fat dependent absorption and bioavailability. Moreover, the microfluidized microparticles of atovaquone cannot easily be formulated into conventional dosage forms like tablet or capsule as those formulation techniques would not allow to maintain the microparticulate matter, leading to agglomerization/crystallization and thus preferably, it is formulated in suspension, rather than inexpensive Tablet/capsule dosage forms.
Nanosupension for enhanced bioavailability is reported in Antimicrob Agents Chemother. 2001 June; 45(6): 1771-1779, however, this also limits its application to micro/nano liquid suspensions for oral administration, and suffers from the above limitation, when to be used in combination with drugs like proguanil. Atovaquone loaded nanocapsules was reported in International Journal of Pharmaceutics Volume 250, Issue 1, 2 Jan. 2003, Pages 273-281, and oily solutions of atovaquone with pluronic surfactants are reported in Journal of Pharmacy and Pharmacology, Volume 58, 2006, Pages 809-820, however, these approaches suffers from commercial applicability because the drug loading concentration in the oil or polymer carrier were extremely low to formulate convenient dosage forms.
Lipid based nanosuspension dispersions of atovaquone are reported by Borhade et al., in Advances in Technology and Business Potential of New Drug Delivery Systems 2011. Solid dispersion of atovaquone-proguanil was also reported under solvent free process condition (ibid), however, the bioavailability reaches only upto 60%, though the aqueous solubility of the formulation is not disclosed. The success of solid dispersion on a polymer carrier reportedly depends on the nature of polymer and its solubility in aqueous solutions. US2008248117, however, reports that solid dispersions has a limitation of increasing solubility of the drug particles beyond the aqueous solubility of the polymer carriers used, as the drug particles are of crystalline in nature, and thus when the polymer coating is leached in the aqueous medium, further solubility is limited by the solubility of crystalline compound in the aqueous solution, and thus aims to form solid solutions of poorly water soluble drugs in polymer carriers, wherein amorphous drug particles are embedded on polymer carriers. The process reported in US2008248117, however, is by short heating of an aqueous suspension of poorly water soluble drugs in suitable polymer carrier under high temperature and pressure, followed by rapid drying to form solid solutions. Although this process forms solid solutions with some of the molecules illustrated, characterized to be amorphous under powder X-Ray diffraction, however, a generalization of the process to get solid solution of all poorly soluble drugs is questionable for following reasons: Water is used as a solvent, and the drug must dissolve in aqueous solution at least for a short time at the high temperature and pressure condition to form amorphous, failing which will result in crystalline nature of the drug. The drug must also be able to remain in the amorphous nature, at least during the period of processing, failing which a rapid phase change to crystalline nature may result, and such changes in the nature of the active agent are likely to provide variability in the dissolution rate.
This aspect is further confirmed by the enhancement in the solubility of the drugs tested in US2008248117:
Resistance toRelease afterRelease afterProductcrushing15 min in %30 min in %Example 12256299Comparative example 11744280Theophylline crystals1403478
Release afterRelease afterProduct15 min in %30 min in %Example 255101Comparative example 23566Carbamazepine crystals2253
The results show that the dissolution rates of the tested drugs increase only marginally to either solid dispersion or the crystals of the active drugs, and the best results show that the solubility is doubled in comparison with the crystals of active drug, for example Carbamazepine. The poor aqueous solubility of the solid solutions of US20080248117 may be attributed to microcrystalline nature of the active drug, which is not been able to be characterized by powder X-Ray diffraction, and powder XRD pattern resembles that of amorphous compound primarily due to the interference of amorphous hydrophilic polymer, and thus fails to characterize microcrystalline nature of active drug, but reflects in poor aqueous solubility. And thus this formulation fails to improve the solubility of compounds such as atovaquone significantly. Moreover, Atovaquone is so far not reported to exist in amorphous form, as processes like lyophilization leads to crystalline atovaquone (ref: WO2009007991), therefore the solution formed for a short period and rapid drying, will allow atovaquone to be precipitated in crystalline form.
It is also worth noting that the dissolution media used for atovaquone is 40% isopropanol buffered to pH 8.0 with potassium dihydrogen phosphate, and product dissolution kinetics are measured in a fixed volume of the dissolution medium, agitated by means of a standardized device; however, the use of organic solvents as dissolution medium neither shows the true picture of in-vivo dissolution, nor shows aqueous solubility. Regulatory authorities in all territories strongly discourage the use of organic solvents in in-vitro dissolution experiments for simple reason that they do not reflect a simulation of in-vivo conditions. Probably atovaquone is an exclusive drug which is recommended with use of organic solvents for which any other aqueous physiologically acceptable dissolution medium was failed to show the required drug dissolution.
To improve the dissolution profile of Atovaquone and its bioavailability, thereby reducing the dose requiring to be administered, it would be advantageous to increase its aqueous dissolution so that it could attain a level close to 100%. Since, Atovaquone is known to be a class II drug according to BCS classification, increasing the aqueous dissolution rate may directly yield in increased absorption and bioavailability.
Thus, there is a need to improve Atovaquone bioavailability by attaining, a level close to 100% dissolution in aqueous dissolution medium, thereby greatly influencing pharmacokinetic effects, bioavailability and therapeutic efficacy with minimized Interindividual variability. This forms the object of the present invention.